Fuel Evaporation Control on Models With Catalytic Converter

Fuel Evaporation Control on Models with Catalytic Converter:

Fuel Evaporation Control:

The fuel evaporation control line of the fuel tank is connected to the activated carbon filter, in which the fuel vapors which accumulate in the tank are collected. The activated carbon filter is linked by means of two further lines to the air manifold. A fuel evaporation control valve is installed in these lines. When the fuel evaporation control valve is open, the vacuum in the air manifold draws in fresh air to the activated carbon filter. The fresh air flushes out the fuel collected in the filter and routes it to the engine, i.e. to the combustion process.

Since this additionally supplied mixture influences combustion to a considerable extend, the fuel evaporation control valve consists of a non-return valve and an electrically operated valve. Due to the non-return valve, the fuel evaporation control valve is closed initially when no power is applied. The non-return valve prevents fuel collecting in the air manifold when the vehicle is parked. This non-return valve opens as vacuum increases in the air manifold. Electrical activation takes place dependent on engine speed and load. The venting cycle (flushing phase) begins as soon as the emission (lambda) control system is active. On completion of one cycle, the valve is closed for approx. 1 min. (rest phase).

This function can only be activated below an engine speed of approx. 500 rpm.

Adaptations:

The fuel-air mixture formed in the intake tract requires a certain period of time until it reaches the oxygen sensor in the form of exhaust gas. This time decreases as load and engine speed increase. For this reason, the response time of the emission control system is also dependent on load and engine speed. Mixture deviations detected by the oxygen sensor result in storage of adaptation values (learned correction values). By way of these adaptations, the injection can be set close to the nominal values in advance, thus achieving a reduction in the response time.

For instance, if the basic injection values for the DME characteristic map are too low during idling in order to maintain the ideal fuel-air mixture, the emission control system would have to constantly increase the injection timing. In this case, an adaptation value is learned which corrects the basic injection value. The emission (lambda) control then only needs to undertake the fine adjustment.

Following adaptations are performed during engine operation:

Fuel Ev8poration Control Adaptation:

When the fuel evaporation control valve is open, an additional combustible mixture or air is supplied to the engine from the activated carbon filter. The shift in the air-fuel ratio detected by the oxygen sensor is almost completely compensated by way of the fuel evaporation control adaptation value.

Idle Air Adaptation:

The EML undertakes idle air adaptation. By way of the throttle valve, it ensures constant idle speeds.

Idle Mixture Adaptation:

If the idle signal is applied during the rest phase of the fuel evaporation control valve, idle mixture adaptation takes place at certain intervals.

Part Load Mixture Adaptation:

Also in the part load range, mixture adaptation takes place at certain intervals. The determined adaptation value is taken into account in all part load ranges.

The correction values stored in the control unit are intended as additional information only In the case of acute engine problems.

If problems occur during engine operation and no defect codes are stored in the defect code memory, the mixture adaptation values can be read out on the BMW Service Tester. These values can provide an indication as to which direction troubleshooting is to be continued.

The adaptation values are lost (forgotten) when the voltage supply to the DME is interrupted. This can lead to starting and idle running problems. Depending on how far the adaptation values deviated from the basic values, a considerable period of time is required until the adaptation values are relearned.

For this reason, idle operation or test runs are of little use In order to observe the change in the adaptation values.